The time duration (henceforth “duration”) of a transmitted signal in a communication system is defined or determined, at least in part, based on the purpose of the signal. A receiver can use the signal properly if it is received in its whole duration. Note that if the receiver receives a signal in only a part of its duration, the receiver may still use the received part for the purpose of the signal. However, the performance, quality or success rate or the operations that are the purpose of the signal will typically be degraded, compared to if the signal was received in its whole duration. For some types of signals, it may be acceptable to receive only a part of its duration. For such signals, a missing portion can be received later in time to complete the processing in accordance with the purpose of the signal. For a non-limiting example, some signals are split into multiple different signal parts that are separated in time. Repetitive signals may be split in multiple different repetitive signal parts. In one non-limiting example, a receiver uses the multiple signals parts to perform the operations that are the purpose of the signal. Therefore, the signal duration spans the multiple signal parts. However, for some other signals, a missing part cannot be received at a later time to complete processing in accordance with the purpose of the signal. Such signals must be received in its whole duration to be properly used.
In some cases, a receiver may receive multiple signals in their whole durations, in order to further improve the performance, quality or success rate of the operations that are the purpose of the signal. For a non-limiting example, the multiple signals can be received in different time periods, different frequencies, different code spaces (using different codes of some kind) or a combination of these. For a non-limiting example, an Long-Term Evolution (LTE) user equipment (UE) may receive multiple primary synchronization signals (PSSs) in their whole durations in different subframes in order to improve detection performance, for example. In the PSS example, a UE combines multiple identical signals. In another LTE example, several (up to 6, for example), LTE positioning reference signals (PRSs) may be transmitted in consecutive subframes, making it possible for UEs to improve the measurements for positioning by combining these PRSs in their whole durations. In another LTE example, a UE may combine multiple different signals in their whole durations, for example when a physical downlink shared channel (PDSCH) is retransmitted using a different redundancy version.
In some cases, a transmitter may transmit multiple signals simultaneously (in parallel), wherein the signals are of the same kind. For a non-limiting example, an LTE base station (eNB) may simultaneously transmit a different cell-specific reference signal (CRS) or channel state information reference signal (CSI-RS) from different antenna ports. A signal as defined above is typically transmitted continuously during its duration, i.e. with positive power, as for instance a primary synchronization signal (PSS) or a secondary synchronization signal (SSS). However, this is not necessary as some signals contain time-periods with zero transmit power, such as PRS and CSI-RS signals in LTE. Here are a few non-limiting examples of signal durations in LTE:                Primary Synchronization Signal (PSS) has the duration 1 LTE OFDM symbol. It is transmitted by an eNodeB (eNB). If a receiver (in LTE called a UE) receives a PSS in its whole duration, it can properly perform the operations that are the purpose of the PSS, e.g. detection, rough time/frequency synchronization and parameter estimation.        Secondary Synchronization Signal (SSS) has the duration 1 LTE OFDM symbol. It is transmitted by an eNB. If a UE receives an SSS in its whole duration, it can properly perform the operations that are the purpose of the SSS, e.g. determination of the physical cell id (PCI).        Cell-specific Reference Signal (CRS) (corresponding to an antenna port) has the duration 1 LTE OFDM symbol. It is transmitted by an eNB. If a UE receives a CRS in its, whole duration, it can properly perform the operations that are the purpose of the CRS, e.g. channel estimation.        Positioning Reference Signal (PRS) has a duration of 11 LTE OFDM symbols. Its power during a few of these OFDM symbols is zero. It is transmitted by an eNB. If a UE receives a PRS in its whole duration, it can properly perform the operations that are the purpose of the PRS, e.g. time of arrival estimation.        Channel State Information Reference Signal (CSI-RS) has a duration of 2 (if 1 or 2 antenna ports) or 9 OFDM symbols (if more than 2 antenna ports), depending on the configuration. In the case of a 9 OFDM symbol CSI-RS signal, only the first two and last two of these have non-zero power. It is transmitted by an eNB. If a UE receives a CSI-RS in its whole duration, it can properly perform the operations that are the purpose of the CSI-RS, e.g. computing a channel quality indicator (CQI).        Physical Broadcast Channel (PBCH) has the duration 4 LTE OFDM symbols. It is transmitted by an eNB. A PBCH carries system information. If a UE receives a PBCH in its whole duration, it can properly perform the operations that are the purpose of the PBCH, e.g. successful extraction of the system information.        Physical Downlink Shared Channel (PDSCH) has the duration 1 LTE subframe (minus the first 1-3 LTE OFDM symbols used for PDCCH/PCFICH/PHICH). It is transmitted by an eNB. A PDSCH carries data (in one or more transport blocks) to a UE. If a UE receives a PDSCH meant for the UE in its whole duration, it can properly perform the operations that are the purpose of the PDSCH, e.g. successful extraction of the data.        Discovery Signal (DS) on unused resource elements (REs) next to PSS/SSS has the duration 2 OFDM symbols, wherein the signal is transmitted by an eNB. If a UE receives a DS in its whole duration, it can properly perform the operations that are the purpose of the DS, e.g. successful discovery of a small cell.        
The LTE examples listed above and below are given for LTE frequency division duplexing (FDD) with normal cyclic prefix (CP). For other LTE configurations, other numbers may apply.
For some signals, especially multi-purpose signals, there are multiple interpretations of the duration, depending on which purpose of the signal that is considered. In the example of LTE CRS, some operations, such as channel estimation, may only require the reception of a single OFDM symbol containing CRS. Other operations, such as frequency offset estimation, may require the reception of multiple OFDM symbols containing CRS to function properly.
In communication systems, signals may be transmitted repetitively in their whole duration. For a non-limiting example, the same signal can be transmitted repeatedly, such as PSS in LTE, which is transmitted during 1 OFDM symbol every 5th subframe. For a CSI-RS in LTE, its transmission period is configurable to between 5 and 80 ms. In another example of repetitive signal transmission, the same kind of signal is transmitted repeatedly, but with some variation in consecutive repetitions. For a non-limiting example, the LTE PBCH is transmitted every 10 subframes, but with four different redundancy versions transmitted in four consecutive transmissions. Hence, the same signal is transmitted every 40 subframes, assuming the system information has not changed. Another example is CRS in LTE, which (for an antenna port) is transmitted every few OFDM symbols, but using different symbol values in different OFDM symbols, where the same CRS symbol values are repeated every 10 subframes. A third example is SSS in LTE which is transmitted every 5 ms, but with only every second SSS being the same signal.
In some communication systems, a signal can be transmitted repetitively such that there is some time between the end of one transmission and the beginning of the next transmission, as in the examples above. A signal can also be transmitted repetitively such that there is no time between the end of one transmission and the beginning of the next transmission. A signal can also be transmitted repetitively in a combination of the two ways just mentioned. For a non-limiting example, LTE PRS signals can be transmitted in a burst (also called a positioning occasion) with PRS in up to 6 consecutive subframes. Such PRS bursts could then be transmitted periodically with a configurable period of 160-1280 subframes.
In some communication systems, a receiver may receive signals only in certain time gaps, here called reception gaps. The reception gaps can be configurable, fully or partly, for example by the network. The reception gap properties can be static, fully or partly, for example as specified in a communication standard, such as LTE. The reception gaps can be periodic. A reception gap configuration could for example be a gap length, period and time offset, which means that a reception gap with the gap length is repeated with the gap period and with a time offset in relation to a reference time. In another example, a reception gap configuration is a pattern of gaps that is repeated periodically. A reception gap configuration could be valid for one or multiple (frequency) carriers. A receiver could be configured with multiple reception gap configurations, for instance one configuration for one carrier and another configuration for another carrier. A receiver could be configured with a reception gap configuration that is valid for multiple carriers, and even for multiple radio access technologies (RATs). Different receivers can have different receiver gap configurations. A receiver can also have multiple reception gap configurations that are valid for the same carrier, with individual properties, such as period, length and offset.
In some communication systems, an LTE UE can be configured to perform inter-frequency cell search, measurements, etc., in certain measurement gaps. During a measurement gap, a UE does not have to receive signals on the serving cell. A single measurement gap in LTE is 6 ms (i.e. 6 subframes) long. The period can be set to either 40 ms, together with a time offset between 0 and 39 ms, or 80 ms, together with a time offset between 0 and 79 ms. The measurement gap configuration is UE specific, which means that different UEs may have different periods and offsets.
An irregular transmission of a signal on a carrier means that it is transmitted so infrequently or sparsely that there is a risk that a UE with a receiver gap configuration for the carrier doesn't receive a signal in any of its reception gaps. In some embodiments, signal transmission irregularity is only considered for signals targeting multiple receivers, such as reference signals, synchronization signals, discovery signals or broadcast channels. In some embodiments, a reason for irregular signal transmission is to save power. In some embodiments, a reason for irregular signal transmission is to reduce interference.
In some communication systems, e.g. LTE, it is possible to configure a receiver to receive signals only in certain reception gaps. During the other times, the receiver may be turned off. The purpose of such a scheme could be to reduce the receiver power consumption. In LTE, this is called discontinuous reception (DRX). Some receivers can be expected to receive signals only during their configured reception gaps.
Irregularly transmitted signals are transmitted so infrequently that there is a risk that a receiver with reception gaps, defined for instance by measurement gaps and/or DRX, does not receive the irregularly transmitted signal at all. In the example of LTE, the regular PSS/SSS periodicity is 5 ms, i.e. 320 times in 1.6 seconds. In an irregular PSS/SSS transmission, they could be transmitted less frequently, for instance only 16 times in 1.6 seconds. Then there is a risk that a UE with measurement gaps doesn't receive such an irregularly transmitted PSS/SSS in any of its gaps. Also, there is a trade-off between signal transmission irregularity and receivers receiving a signal often. In fact, for some combinations of irregular signal transmission and reception gap configurations, some receivers may never receive the signal at all. For example, consider a signal transmitted during 1 ms every 1600 ms and a receiver with a periodic reception gap with a period of 80 ms, a gap length of 6 ms and a gap time offset between 0 and 79 ms. In this example, the receiver may receive the signal in every 20th reception gap, if the gap time offset matches the irregular signal transmission, i.e. if the irregular signal transmission occurs within a reception gap. For all other gap time offsets, the receiver will not receive the signal at all, in any reception gap.